Is this normal...

When coasting at around 40mph or less, my tach dips down to about 700 rpm then back up to around 1200.

Probably a stupid question, but it's my first AT car and not sure if this is normal. I know in a manual the tach never dips unless the car is put into neutral. The BMW 5-series auto doesn't seem to do this either. Thanks!

 

That's probably an auto downshift from 5th to 4th.

Find someplace you can let the car drift from 50-0 and watch the tach for downshifting.

 

What you're observing may also just be the fluid clutch doing its thing, which is: slipping.

 

If you're coasting down after a steady state cruising near 40mph and you're in 'D' or have MM in '5', you're tach should not drop right away. The Torque Convertor will stay locked until 1,300rpm and then slowly drop and rise as it downshifts gears.